7 Weeks Scan Advice - worried...

Hi, I'm looking for some advice or anyone with similar experience. I am expecting 1st baby, I am around 7 weeks but have very irregular periods so exact date is unsure. I have had 3 scans so far as I have history of PCOS and lack of ovulation. Had scan two weeks ago and baby was dated 5 weeks but no heartbeat - said it was just too early. Went back today, two weeks since last scan, baby only measuring 6 and a half weeks. Midwife said she could see a heartbeat but at times it looked a little slow. Other times it looked normal but she said maybe baby was just too small to tell yet. I have to go back in a week for another scan but I am so worried something is wrong or I might miscarry my baby if heartbeat isnt as fast as it should or is too small. I feel like I just don't know what is normal and what isn't and have been so anxious. Would appreciate any advice or if anyone has had anything like this? Thanks!